The present invention relates to the activation of normally nonconductive surfaces for electroless plating.
Electroless plating is the most important commercial route providing the surface of a nonconductor, such as molded polymers, with a tenacious coat of metal. The important properties of each material are combined. Products may be decorative as well as functional.
In its commercial aspects, an electroless plating operation consists of a number of batchwise operations in series through which articles to be electrolessly plated are passed with rinse operations between each stage. Although residence time will vary from stage to stage, the cycle time for transport apparatus employed requires a degree of coordination of residence time as between the several functional stages involved.
Three principal steps are involved. They are surface etching, surface activation and electroless plating itself. Depending on end use, electroless plating may be and usually is followed by electrolytic plating.
Surface etching involves the immersion of a substrate to be electrolessly plated, i.e. a plastic, into a strong oxidizing acid to etch or "microcraze" the surface to render it porous. This may be preceded by contact with a suitable organic etch to deglaze the surface of the article. A preferred acid etch is that described in U.S. Pat. No. 3,366,130 issued to two of us on June 6, 1972.
Surface activation involves seeding of the porous surface with a metal active to the deposition, through reduction, of a metal contained in the electroless plating bath. To date surface activation, by absorption from aqueous systems has only been possible with the noble metals, e.g. palladium and platinum. The noble metal most typically used is palladium. Seeding is followed by electroless plating using solutions well known in the art. The solutions are typically copper, nickel or cobalt based.
Existing commercial methods of depositing a noble metal involve two distinct stages aside from any rinse operations which may be employed. One method involves contacting the article with a sensitizing solution containing stannous chloride, followed by immersion in an activator solution such as palladium chloride where Pd.sup.+.sup.+is reduced to Pd.sup.o on the surface of the article
Another employs a highly acidic solution of a noble metal colloid, typically a palladium colloid, maintained in suspension by a protective colloid, i.e. stannic acid colloids. Activating solutions of this nature are described in U.S. Pat. No. 3,011,920 to Charles R. Shipley, Jr.
In their preparation, a stannous salt, such as stannous chloride is added in an amount greatly in excess of that required to reduce Pd.sup.+.sup.+ to Pd.sup.o to contemporaneously form the protective colloid.
Colloidal palladium formed by other reductants and protected by other protective colloids under alkaline conditions are also mentioned. They are, however, stated to be less effective than the acidic colloids and, as presently known, have not been used to any extent.
Because the colloidal palladium as well as the protective colloid are co-absorbed by the substrate to be electrolessly plated, the article is immersed in an acidic or alkaline accelerator solution to remove the protective colloid and expose the absorbed noble metal.
The acceleration step can be eliminated if the electroless plating solution is highly alkaline in nature and capable of removing the protective colloid. This route, however, is not commercially feasible since prolonged immersion times are required before electroless plating begins. This method, therefore, also requires, in substance, two stage activation.
Because of the high cost of noble metals and certain problems attendant to their use, it would be desirable to seed a surface with another material such as the metal contained in the electroless plating bath or one which is receptive to the metal contained in the electroless plating bath.
However, no aqueous solution or dispersion of metals other than the noble metals are presently known to be capable of permitting absorption of metals onto the surface of a nonconductor so as to render the seeded surface catalytically active for electroless plating.
A suggested route has been to finely grind a metal receptive to the metal contained in the electroless plating solution and blend the metal with a resin prior to molding. The basic theory is that the metal will be exposed on the surface of a molded part to permit electroless plating. In addition to being impractical, bond strengths and plate are non-uniform which defeats one requirement of a truly flexible electroless plating operation. That is, the ability to achieve a tenacious coat of metal of sufficient uniformity to permit a subsequent electrolytic plating operation to form a "mirror" finish on an article.